Ukraine's interim prime minister is appealing for international support - to bail out Ukraine financially and save it from Russian aggression. He's making his case at the U.N. today, just days before part of his country, Crimea, is expected to vote to join Russia. Ukraine and its backers in the West say that referendum is illegal, it violates international norms. But time is running out to persuade Russia to back off.

NPR's Michele Kelemen has the story.

MICHELE KELEMEN, BYLINE: The young technocrat who now leads Ukraine's interim government says there's much at stake in this current crisis.

ARSENIY YATSENYUK: It's all about freedom and we want to be very clear, we will never surrender.

KELEMEN: Arseniy Yatsenyuk says he wants a diplomatic solution and needs the world's help. He calls this a global crisis and adds much will depend on one man, Russian President Vladimir Putin.

YATSENYUK: You probably do remember his speech a few years ago saying that the biggest disaster of the last century is the collapse of the Soviet Union. I will say that the biggest disaster of this century would be the restoring of the Soviet Union.

KELEMEN: Yatsenyuk says he's satisfied with the support he's getting so far, including from President Obama, who made clear in their Oval Office meeting that the U.S. won't recognize this weekend's referendum in Crimea.

PRESIDENT BARACK OBAMA: We completely reject a referendum patched together in a few weeks with Russian military personnel basically taking over Crimea.

KELEMEN: Obama says the U.S. recognizes the Kremlin's ties to the region, where Russia's Black Sea fleet is based, and Ukrainian authorities say they're ready to work with Russia on that. But Russia doesn't recognize the new Ukrainian government - who it says came to power in a coup. Obama is sending Secretary of State John Kerry to London to meet again with his Russian counterpart to see if there's any diplomatic way ahead.

OBAMA: We do not know yet what this diplomatic efforts will yield, but we will keep on pressing.

KELEMEN: A former White House advisor on Europe, Damon Wilson, says if diplomacy is to have any chance, the U.S. has to make clear there will be costs.

DAMON WILSON: Vladimir Putin probes. He tests. He gauges how much he can get away with. We have to stand up to this aggression. And we have to make sure that Putin feels the pain, feels the cost. He's playing with fire and he needs to have his hands burned in this process.

KELEMEN: There's always a chance that Putin could keep Crimea in legal limbo, even after Sunday's referendum. But Wilson, who's with the Atlantic Council, argues that the U.S. and the European Union should still show resolve and move ahead with plans to impose targeted sanctions.

WILSON: Because regardless of the formalities of it, he has dismembered Ukraine. He has seized a chunk of its territory. He has positioned himself to maintain leverage over that country as he tries to prevent it moving to the West and he's really trying to make Ukraine an unattractive partner to the European Union, to NATO. What do you do with a country with an occupied territory, with troops on its territory?

KELEMEN: Wilson hosted Prime Minister Yatsenyuk at the Atlantic Council and the two talked about the challenges ahead and the need for Western support. Yatsenyuk calls his job political suicide, but says he's determined to get Ukraine's fiscal house in order and carry out reforms needed for an IMF bailout and closer ties with the European Union.